Abstract: The most load of the arc dam is transmitted to the abutment rock mass of both sides through the arch dam foundation. How to accurately evaluate the quality of dam foundation rock mass during construction period is important for engineering geology research of arch dam foundation rock mass. Based on the engineering geological conditions, the authors studied the engineering geological characteristics of foundation rock mass, such as lithology and rock mass structure etc., and built the rock mass quality classification standard, combining with the field and laboratory tests and the sonic test of rock mass. According to the rock mass quality classification during excavation, most of the rock mass is of gradeⅡ, a small amount of grade Ⅲ₁, and very small amount of Ⅲ₂, which are agreed with the result of the feasibility study stage. The quality of rock mass meets the requirement of dam construction. The distribution of rock mass is continuous and beneficial to the arch dam.

Key words: Wudongde hydropower station, high arc dam, dam foundation, rock mass quality classification, construction period